Modified membranes

ABSTRACT

A porous polymeric membrane formed from a blend of a polymeric membrane forming material, such as polyvinylidene fluoride or polysulfone and a polymeric reactivity modifying agent adapted to modify the surface active properties of the porous polymeric membrane. The reactivity modifying agent is preferably a linear polymeric anhydride, such as poly(alkyl vinyl ether/maleic anhydride). The surface activity modifications include modification of the hydrophilicity/hydrophobicity balance of the membrane, or hydrolysis followed by reaction with a polyamine to form a crosslinked polyamide layer. Such modified membranes have use as reverse osmosis membranes.

TECHNICAL FIELD

The invention relates to porous membranes which have modified physicalproperties imparted by the addition of chemical modifiers.

BACKGROUND ART

Synthetic membranes are used for a variety of applications includingdesalination, gas separation, filtration and dialysis. The properties ofthe membranes vary depending on the morphology of the membrane ieproperties such as symmetry, pore shape and pore size and the polymericmaterial used to form the membrane.

Different membranes can be used for specific separation processes,including microfiltration, ultrafiltration and reverse osmosis.Microfiltration and ultrafiltration are pressure driven processes andare distinguished by the size of the particle or molecule that themembrane is capable of retaining or passing. Microfiltration can removevery fine colloidal particles in the micrometer and submicrometer range.As a general rule, microfiltration can filter particles down to 0.1 μm,whereas ultrafiltration can pass through particles as small as 0.01 μm.Reverse Osmosis operates on an even smaller scale.

As the size of the particles to be separated increases so too does thepressure required to carry out the separation and the density of themembrane.

A large surface area is needed when a large flux is required. One knowntechnique to make filtration apparatus more compact is to form membranesin the shape of a hollow porous fiber. Modules of such fibres can bemade with an extremely large surface area per unit of volume.

Microporous synthetic membranes are particularly suitable for use inhollow fibres and they are produced by phase inversion. In this process,a polymer is dissolved in an appropriate solvent and a suitableviscosity of the solution is achieved. The polymer solution can then becast as a film or hollow fiber, and then immersed in a precipitationbath such as water. This causes separation of the homogeneous polymersolution into a solid polymer and liquid solvent phase. The precipitatedpolymer forms a porous structure containing a network of uniform pores.Production parameters that affect the membrane structure and propertiesinclude the polymer concentration, the precipitation media andtemperature and the amount of solvent and non-solvent in the polymersolution. These factors can be varied to produce microporous membraneswith a large range of pore sizes (from less than 0.1 to 20 μm), andaltering chemical, thermal and mechanical properties.

Microporous phase inversion membranes are particularly well suited tothe application of removal of viruses and bacteria. Of all types ofmembranes, the hollow fiber contains the largest membrane area per unitvolume.

Different techniques can be used to induce phase separation and preparepolymer membranes. A polymer dissolved in a solvent can solidify uponcooling, which is known as liquid-solid phase separation. Phaseseparation can be induced either by a temperature change or by a changein the concentration of the solution. These two processes are referredto as thermally induced phase separation (TIPS) and diffusion inducedphase separation (DIPS). The morphology induced by phase separationneeds to be secured and hence solidification of the polymer phase needsto be achieved. In the TIPS process this is usually done by dropping thetemperature below the glass transition temperature or the melting pointof the polymer. The DIPS process uses a change in concentration, causedby diffusion of a solvent and a non-solvent, to induce a phaseseparation. With this technique, the hollow fiber membranes are producedusing a batchwise process. The DIPS process has an advantage thatasymmetric membranes can easily be formed. In addition, the spinning ofhollow fibers can be performed at room temperature, whereas thealternative process—thermally induced phase separation (TIPS) requiresmuch higher temperatures. Since DIPS uses the diffusion of non-solventand solvent it is relatively easy to control the rate at which membraneformation takes place by changing the concentration of the non-solventbath and the polymer solution. The disadvantage however, is thatmacrovoids can be produced, in the form of fingerlike intrusions in themembrane. They decrease the mechanical strength of the membrane but canbe avoided by choosing the right composition of solution.

Flat sheet membranes are prepared in the following way. A polymersolution consisting of a polymer and solvent is brought into contactwith a non-solvent. The solvent diffuses outwards into the coagulationbath and the non-solvent will diffuse into the cast film. After a givenperiod of time, the exchange of the non-solvent and solvent hasproceeded such that the solution becomes thermodynamically unstable anddemixing occurs. Finally a flat sheet is obtained with an asymmetric orsymmetric structure

Hydrophobic surfaces are defined as “water hating” and hydrophilicsurfaces as “water loving”. Many of the polymers that porous membranesare made of are hydrophobic. Water can be forced through a hydrophobicmembrane by use of sufficient pressure, but the pressure needed is veryhigh (150-300 psi), and the membrane may be damaged at such pressuresand generally does not become wetted evenly.

Hydrophobic microporous membranes are characterised by their excellentchemical resistance, biocompatibility, mechanical strength andseparation performance. Thus, in the application of water filtration,such hydrophobic membranes need to be hydrophilised to allow water topermeate them. Many hydrophilic materials are not suitable for MP and UPmembranes that require mechanical strength and thermal stability sincewater molecules play the role of plasticizers.

Currently, poly(tetrafluoroethylene) (PMT), Polyethylene (SB),Polypropylene (PP) and polyvinylidene fluoride PVDP) are the mostpopular and available hydrophobic membrane materials. Polyvinylidenefluoride (PVDF) is a semi-crystalline polymer containing a crystallinephase and an amorphous phase. The crystalline phase provides goodthermal stability whilst the amorphous phase has flexibility towardsmembranes. PVDF, exhibits a number of desirable characteristics formembrane applications, including thermal resistance, chemical resistance(to a range of corrosive chemicals, including chlorine), and weather(UV) resistance

Modification of a polymer's surface potentially can maintain a polymer'sdesirable bulk properties but can provide new, different interfacialproperties. Membranes made from hydrophilic polymers are generally lessprone to fouling than the hydrophobic polymers. In some instances,surface modification of the more chemically resistant polymers hasrendered them less susceptible to fouling. Numerous techniques exist forthe surface modification of polymers. The most common examples of thischemistry are reactions that introduce a single type of functional groupor mixture of functional groups.

In general, all techniques of hydrophilisation of polymer surfacesinvolve an increase of the surface amount of polar groups. From amicroscopic point of view, the basis of surface hydrophilisation is tomaximise hydration and hydrogen bonding interactions. All sorts ofoxygen, nitrogen or sulfur containing organic functional groups caninteract with water more effectively than common carbon based repeatingunits. There are various methods for wetting a membrane on anon-permanent basis. One method of hydrophilising a porous hydrophobicmembrane has been to pass alcohol through the pores of the membrane,then replace the alcohol by water. Surfactants and a post treatment witha glycerol coating have also been used. This is an adequate solution tothe problem, so long as the water remains in the pores. However, if thewater is removed from the pores either wholly or partially, and they arefilled with air, the hydrophilised membrane is rendered hydrophobicagain, and water cannot pass through the pores if it is not subjected tohigh pressure.

It is an object of the present invention to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative.

DESCRIPTION OF THE INVENTION

According to a first aspect the invention provides a porous polymericmembrane formed from a bland of a polymeric membrane forming materialand an anhydride as a reactivity modifying agent adapted to modify thesurface active properties of the porous polymeric membrane relative to aporous polymeric membrane formed from the polymeric membrane formingmaterial alone.

The term “blend” as used herein refers to an intimate mixture or alloyof the polymeric membrane for material and polymeric reactivitymodifying agent which requires that the two components be compatible, iemiscible, with one another.

Preferably, the polymeric membrane forming material is of low reactivityrelative to the reactivity modify agent. The polymeric membrane formingmaterial may in some cases desirably be inert.

Preferably, the polymeric membrane forming material is PVDF, especiallyhydrophobic PVDF.

In alternative preferred embodiment, the polymeric membrane formingmaterial is polysulfone. The term polysulfone is used herein in thebroad sense in which it is understood by those skilled in the art, andis intended to encompass polysulfone per se, as well as the polyethersulfones, polyaryl sulfones (in particular, polyphenyl sulfone),polyalkyl sulfones, polyaralkyl sulfones and the like.

According to a second aspect, the invention provides a method ofpreparing a porous polymeric membrane wherein an anhydride as areactivity modifying agent is blended in the surface active porouspolymeric membrane by incorporation into the bulk material

By reactivity modifying agent, it will be understood to include, but notlimited to, the overall behaviour of the membrane and/or the membranesurface to chemical species. In addition to reactivity, the membrane maybe modified with respect to other properties. For instance, aparticularly preferred reactivity to be modified is thehydrophilicity/hydrophobicity and/or the surface charge of the membrane.

Preferably, the reactivity modifying agent is a polymeric reactivitymodifying agent, in particular a linear polymeric anhydride. Mostpreferably the polymeric reactivity modifying agent is Gantrez™.

In a first embodiment, the reactivity modifying agent is included in thesurface active porous polymeric membrane by incorporation into the bulkmaterial, for example, by combining with the polymeric membrane formingmaterial prior to or during membrane formation. In particular, thereactivity mowing agent is added to the polymer dope mixture before themembrane is cast.

The reactivity modifying agent may be incorporated into the membrane ina reacted or unreacted form. The reactivity modifying agent incorporatedinto the polymeric porous membrane may be subjected to chemicalmodification subsequent to incorporation into the membrane. Onepreferred chemical modification is hydrolysis to render the membranehydrophilic. Other preferred chemical modifications include crossliking. In a highly preferred form of the invention, the Gantrez™ isincluded in the membrane during formation and the resultant Gantrez™modified membrane is then subjected to crosslinking or an additionaltreatment which modifies reactivity, such as treatment with one or moreof tetraethylenepentamine (TEP), tris-(hydroxymethyl) aminomethane(TRIS), sulfuric acid (H₂SO₄), polyethylene glycol (PEG), calciumcarbonate (CaCO₃).

Membranes containing reactive surface groups, such as acid or anhydridegroups resulting from the incorporation of Gantrez can be crosslinkedwith cross linking amines to form amide linkages. The degree ofincorporation and extent of crosslinking can be used to construct amembrane suitable for reverse omosis operations. In this way, polymericreactivity modifying agents such as Gantrez can form porous polymericmicrofiltration or ultrafiltration membranes which act as substrates forreverse osmosis membranes.

When the reactivity modifying agent is incorporated into the polymericmembrane forming material before or during membrane formation, it ispreferably adapted to combine with the membrane forming material in anamount such that the combined precursor to the surge active porouspolymeric membrane acts as a single phase mixture. The membrane may thenpreferably be prepared by known methods in the art. Most preferably, itis formed into a hollow fibre membrane or a flat sheet membrane.

According to a third aspect, the invention also provides a method ofmodifying the surface of a porous polymer membrane including:

-   i) blending an anhydride as a reactivity modifying agent with a    polymeric membrane forming material and-   ii) forming a modified membrane.

The invention also provides an agent for forming a modified membrane inaccordance with the preceding aspects.

According to a fourth aspect the invention provides a blend of amembrane forming polymer and a compatible second polymer, said secondpolymer being capable of chemical modification after membrane formation.

Preferably, the compatible second polymer is compatible with PVDF, orpolysulfone, or more preferably, both.

The commercial copolymer Gantrez™, or poly(methyl vinyl ether/maleicanhydride), is a linear polymeric anhydride which is available inseveral molecular weight ranges.

Due to the reactive anhydride sites in the Gantrez™ structure and theextensive chemistry associated with this, Gantrez™ can not only providethe appropriate hydrophilicity to the polymer fibers but may providenumerous possibilities for tailoring the fiber to specific applications,depending on the chemistry involved.

In one embodiment of the invention, Gantrez™ was added to the polymerdope mixture and both flat sheet and hollow fiber membranes wereproduced from this. The primary tests carried out on the flat sheetmembranes constituted the initial experimentation, and involvedquantifying the dyes absorbed after post-treatment of the membranes.

The hollow fiber membranes produced were tested for tensile strength,permeability, microscopic structure and the change in glass transitiontemperature of the is modified membrane.

Gantrez™ is a commercial ingredient. Its properties and diversity bothin the hydrolysed and non-hydrolysed forms are known. Gantrez™ allowsmembranes to be fabricated which have excess anhydride sites, and byadding various amines and bases for example, the essential mechanicalproperties of the membranes can be preserved, yet hydrophilicity (andhence flow through the porous membrane) and reaction with othermaterials can be improved. There can also be a resultant decrease infouling of the membrane, since say for instance there was a slightnegative charge on the fiber then more particles would stay in suspendedin solution.

Gantrez™ or poly(methyl vinyl ether/maleic anhydride) is a water solublecopolymer which has the following structure:

Gantrez™ is a linear polymeric anhydride, which is the interpolymer ofmethyl vinyl ether and maleic anhydride. Gantrez™ is manufactured byInternational Specialty Products ISP) and is available in a range ofmolecular weights. The physical and chemical properties of somepreferred forms of Gantrez™ are listed below:

-   Appearance: white, fluffy powder-   Softening point range 200-225° C.

Molecular Weights, Specific Viscosity and Glass Transition Temperature:M_(w) M_(N) Viscosity T_(g) (° C.) Gantrez ™ AN 119 2.13 × 10⁵ 6.19 ×10⁴ 0.1-0.5 152 Gantrez ™ AN 139 8.72 × 10⁵ 2.21 × 10⁵ 1.0-1.5 151Gantrez ™ AN 149 1.25 × 10⁶ 3.53 × 10⁵ 1.5-2.5 153 Gantrez ™ AN 169 1.89× 10⁶ 5.78 × 10⁵ 2.6-3.5 154Poly(methyl vinyl ether/maleic anhydride) having a molecular weight inthe range 5×10⁴ to 5×10⁷ may also be used. The molecular weights weredetermined by size exclusion chromatography, the specific viscositycarried out at 25° C. in a 1% MEK solution in a capillary viscometer.

The two monomer types in Gantrez™ individually contribute properties tomake the copolymer highly valued material in a variety of applications.The poly(methyl vinyl ether) is a flexible film former while the maleicanhydride is a hard polar monomer which contributes to bonding strength.

The reaction of maleic anhydride and methyl vinyl ether is as follows:

Gantrez™ is soluble in water and several organic solvents includingalcohols, phenols, pyridine, aldehydes, ketones, lactams and loweraliphatic esters. It is insoluble in aliphatic, aromatic or halogenatedhydrocarbons, ethyl ether and nitroparaffins.

When the copolymer dissolves in water or alcohols, the anhydride linkageis cleaved such that the formation of the polar, polymeric free acid orthe corresponding partial esters occurs.

Both aqueous and organic solvent solutions of Gantrez™ form films thatare tack-free and possess a high tensile and cohesive strength. Thesefilms do, however, possess an inherent brittleness, so this properlywill have to be carefully monitored when incorporating the Gantrez™ intothe membrane.

Although Gantrez™ powder is hygroscopic and will slowly hydrolyze in ahumid environment, it can absorb up to 30 wt % water without effectingthe flow characteristics of the powder. Mild heating (100° C. for 1-2hours) can remove unbound water.

Several studies have been carried out in making asymmetric membranesusing small molecular additives to improve membrane characteristics.Such characteristics include high permeability, good macroscopicintegrity such as circular lumen, uniform wall thickness, and mechanicalstrength. It is known from literature that PVDF has a small criticalsurface tension (about 25 dynes/cm), the coagulation rate and fibersolidification are slow as a result of the weak interaction between thecoagulant (water or solvent) and the polymer. As a consequence,difficulties arise when preparing porous asymmetric PVDF hollow fibermembranes without additives.

Surface Grafting in polymer chemistry is a process known in the art bywhich side chains of a second polymer are introduced onto an existingpolymer main chain Surface grafting generally leads to an overlayer of asecond functionalized polymer covalently linked to the substratepolymer. Unlike simple functionalisation, which produces a polymersurface or interphase, grafting produces a physically distinct overlayerwith properties that resemble those of the pure graft homopolymer. Themethodology for grafting is based on radical polymerisations.

In Contrast to techniques where the surface composition is modified bysome external treatment, in bulk modifications the surface compositionis the result of the effect of the presence of another component in thepolymer system.

The aim for polymer blends is to combine the properties of the singlecomponents into one material. By dissolving one polymer in anotherpolymer so that there is an interpenetration of one polymer in another.The “polymer blend” that is synthesized in this way is not stablethermodynamically. Thermodynamic instability means that a demixingprocess occurs in the melt. To increase the stability it is necessary tocrosslink one or both polymers.

Surprisingly, it has been found by the present applicants that Gantrez™is miscible with polar polymers such as the polysulfones and PVDF.Normally, very few polymers are miscible with other polymers—forexample, the literature reveals that there is known only one polymer,polyvinyl pyrrolidone (PVP) which is miscible (compatible) with bothPVDF and polysulfone.

In the prior art, it is known to use mixtures of two polymeric membraneforming materials which are not miscible. These prior art mixturesrequire stabilization by cross linking, ie the two non misciblematerials are stabilized by chemical reaction, and in this regard, theyare not true “blends” as defined herein.

Gelation is an important consideration in membrane formation. Gelationcan be defined as the formation of a three dimensional network bychemical or physical crosslinking. When gelation occurs, a dilute ormore viscous polymer solution is converted into a system of infiniteviscosity, that is, a gel. A gel can be considered as a highly elastic,rubberlike solid. It is important to note that gelation is not a phaseseparation and can occur in a homogeneous solution (that is a polymerand a solvent) as well. Experiments will have to be carefully monitoredso as to avoid gelation in the polymer solution, and one of theobjectives of the chemical modification of the pre-existing membranes isto optimize the amount of Gantrez™ in the polymer solution withoutgelation occurring.

Gelation of blends which incorporate Gantrez™ can occur if there aremultivalent components in the polymer blend such as polyalcohols (invarious forms such as sugars, polyvinyl alcohols, polyethylene glycols,ethylene glycol, glycerol and the like). These gelation reactions can beminimized by reducing the time the Gantrez™ spends in solution with themultivalent components by blending the Gantrez™ into the membraneforming mixture (or vice versa) immediately prior to membrane casting.

One of the most capable copolymers at binding molecules are maleicanhydride copolymers. The reactions exemplified below are just some ofthe many reactions that can occur.

As mentioned above, Gantrez™ undergoes hydrolysis quite readily. Thehydrolysis reaction was carried out in the presence of a sodium buffer(0.5M sodium chloride+0.1M sodium borate pH=9.3). The problem withhydrolysing the Gantrez™ lies in the fact that the reactive anhydridesites will be lost when this reaction takes place. Secondly, thesolvents that Gantrez™ does dissolve in will also dissolve the polymermaterial that makes the membrane fibers.

To overcome this problem, in the surface modification trials, theGantrez™ is only partially hydrolysed then reacted with thepolyfunctional amine, in order to crosslink Gantrez™ with the reactiveamine groups. Under the appropriate conditions, the reaction of aminegroups can be favoured over hydrolysis even in an aqueous medium.

A variety of methods are available for the characterisation of surfacesand permeability. One method to study adsorption or fouling onto asurface is to use radioactive labels, tagged to the appropriatemolecule, however this method has proved too cumbersome.Characterisation of the modified membranes is crucial because a smallchange in one of the membrane production parameters can change thesurface structure and consequently have a drastic effect on membraneperformance. Structural membrane properties such as pore size and poresize distribution are able to determine which particles or molecules areretained and which will pass through the membrane.

The two techniques primarily used to observe vibrational spectra areinfrared (IR) and Raman spectroscopy. The IR spectrum arises from theabsorption of radiation, which results from transitions among thevarious vibrational quantum levels. Infrared testing is carried outprimarily on the bulk modified polymer membranes. IR measurements aretaken on a polymer solution with and without the Gantrez™ incorporatedinto the polymer matrix and these two results when compared. One regionof interest in the investigation of Gantrez modified polymericmembranes, are the sharp amide peaks around 1700 cm⁻¹ wavelength.

Attenuated Total Reflectance, also known as Internal ReflectionSpectroscopy is invaluable in the characterisation of surface layers.This technique relies on the intimate contact angle of a sample with thesurface of a high refractive index, IR-transparent prism. The basicprinciples behind this technique is that infrared radiation enters theprism at an angle greater than the critical angle, and is internallyreflected at the prism surfaces, but attenuated by absorptions from thesample contact layer.

Primarily, the SEM is an instrument for the examination of surfaces, andis a very convenient, simple technique for characterizing andinvestigating the porous structure of membranes. The resolution limit ofa simple electron microscope lies in the 0.01 μm range, with moresophisticated microscopes operating at resolutions of 0.005 μm (5 mm). Aclear and concise picture of the hollow fiber membrane can be obtainedin terms of the surface and cross-section, and an estimation of theporosity and pore size distribution can also be obtained.

Many polymers are poor conductors of electricity, and as a result,charge rapidly builds up on the surface of the sample as the electronbeam is scanned across it. The resulting field then interacts with theincident electron beam and causes image distortion. This problem can beovercome by coating the sample with a conducting layer usually gold. Thegold is applied by sputtering and typical film thicknesses are 20 nm.Sputtering involves creating ions, accelerating them on a target,forming atoms or clusters which are then deposited on the membranesubstrate.

The bubble point method is a reflection of the maximum pore size. It isthe force needed to drive a liquid through the pores. The liquid used inthis case is water, and the gas pressure at which a bubble emerges ismeasured. The maximum pore size can be calculated from bubble point.

Mechanical tests are often used to assess the ageing or chemicalresistance of materials. The change in tensile properties of a material,are useful indicators of the degradation of a material. Testing consistsof securing the test sample between two sets of grips. One set of gripsis fixed and the other is attached to a moving crosshead and load-cellarrangement. Machines measure the force necessary to elongate and breakthe sample.

The break force is measured and reported as a tensile stress value bydividing the force obtained by the cross-sectional area:Tensile stress (Mpa)=F/A

Where F=force (Newtons) required to break the test piece, andA=cross-section area of test piece (mm²). The break force is areflection of the strength of the polymer fiber and is obviously of highimportance in determining performance of the membranes.

Break extension is the elongation measurement from the tensile machineis given by the extension in gauge length divided by the original gaugelength. Break Extension is given as a percentage figure, while strain isshown as a fraction.

-   Tensile strain=change in length/original length=(l₁−l₀)l₀-   Where l₁=length between gauge marks (mm) and l₀=original gauge    length (mm)-   Break Extension=(l₁−l₀)l₁×100%-   The Break Extension is a measure of the elasticity of the polymer    fiber, which can also be expressed as Young's modulus or the modulus    of elasticity:-   Young's modulus=Stress/strain in linear portion of stress-strain    curve.    This is the ratio of the applied stress to the strain it produces in    the region where strain is proportional to stress. This modulus is    primarily a measure of stiffness. Obviously, as the polymer    degrades, the break extension will decrease, so these two tests are    excellent measures of polymer degradation.

Permeability is a primary factor in governing the performance orefficiency of a hollow fiber membrane for water filtration applicationsis the flow or flux through the membrane. The flux or permeation rate isdefined as the volume flowing through the membrane per unit area andtime. The equation that describes the flux is:J=Q/AΔt

Where Q is the permeated amount, A is the membrane area and Δt thesampling time. The permeability is an important parameter whenconsidering the effect that the addition of Gantrez™ has on the membraneperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the time required for complete hydrolysis of Gantrez™ as afunction of temperature.

BEST MODE FOR CARRYING OUT THE INVENTION

Experimental Methods

Production of Flat Sheet Membranes

A dope solution was prepared according to the following formulation:PVDF  17.1 g (17%) LiCl  3.54 g (3.5%) PVP/VA S630  2.73 g (2.7%)Gantrez  3.09 g (3.1%) NMP  74.3 g (73.7%) 100.76 g

-   PVP/VA S630 is polyvinylpyrrolidone/vinyl acetate copolymer-   NMP is N-methyl pyrrolidone

All ingredients were added together and the dope solution was allowed tomix on heated rollers (50° C.) overnight. Once removed from the rollers,the mixture was left to settle (to remove any bubbles from the solution)for half a day.

The dope solution was cast onto a glass plate (which was notablyhydrophilic) using a glass rod and a membrane was cast by immersing theglass plate into a precipitation bath consisting of:

-   -   45% PEG-200    -   45% H₂O    -   10% NMP

The membrane was immersed in the precipitation bath for 10 minutes(until the membrane was able to be “peeled off” the glass plate).

The membrane was divided into equal proportions and samples were soakedin the following post-treatment solutions until testing was carried out:

-   1. 3% Tetra ethylenepentamine CMP)-   2. 3% Trishydroxymethyl) aminomethane(I)-   3. 1% Sulfuric Acid (H₂SO₄)-   4. 100% Polyethylene glycol (PEG)-   5. 100% Calcium Carbonate (CaCO₃)-   6. 100% Butanol

A second flat sheet dope formulation was produced to observe themembrane without the additive PVP/VA S630. The following solution wasmade up: PVDF  17.04 g (16.8%) Gantrez ™  3.44 g (3.4%) LiCl  3.88 g(3.8%) NMP  77.02 g (76%) 101.38 g

As before, the membrane was cast in a precipitation bath of PEG/H₂O/NMP,and samples immersed in the six solutions mentioned above. Thequalitative and quantitative analysis were carried out as above andresults are detailed in the next section.

Analysis of the Flat Sheet Membranes

Both qualitative and quantitative analysis was carried out on thedifferent membranes. The qualitative analysis that was carried outinvolved dipping 1 cm² samples of membrane into different coloured dyes,namely Saffranin, Methyl Orange and Fuchsin. The intensity of thecolours in the membranes was examined visually by taking photos of themembranes after they had been in the dye solutions for a day. Thesamples were then placed in water, and photos were taken of the amountof colour retained and lost by the membranes. The results are outlinedin the next section.

The quantitative analysis involved the measurement of the intensity ofthe colour retained by the samples using a Hach Spectrophotometer.

Initially, the wavelengths of the dyes had to be calculated using atrial and error method Because the region of wavelengths of the coloursis known (for example red is between 500 and 600 nm) the exactwavelength could be calculated. The wavelength corresponding to themaximum absorbance obtained, was the optimum wavelength of the specificcolour.

The calibration for the range of concentrations of colours was thencarried out. A graph of set concentrations versus measured absorbanceswas plotted. This was subsequently used to read off the graph unknownconcentrations, or to use the calibration equation to calculate unknownvalues.

Production of Hollow Fiber Membranes

The following dope solutions were used for the production of hollowfiber membranes: All weights are in grams. Dope 1 Dope 2 Dope 3 Dope 4PVDF  119.3 (17%) 103.76 (17.2%) 102.49 (17%) 102.61 (17%) Gantrez 21.44 (3.1%)  18.43 (3.1%)  18.35 (3.1%) S630  18.62 (2.7%)  16.05(2.7%)  15.93 (2.65%)  15.66 (2.6%) LiCl  21.09 (3%)  18.02 (3.0%) 18.32 (3.05%)  18.66 (3.1%) NMP  520.6 (74.2%) 446.49 (74.1%)  464.6(77.3%) 446.61 (74.2%) Total 701.05 602.75 601.34 601.89

The generic brand of PVDF that was used for Dopes 1-3 was Kynar 461,whereas Kynar 500 was used for dope 4, which has a higher molecularweight than Kynar 461. In the case of Dope 1 was all mixed together onheated rollers for a day and half Dopes 2-4 were mixed such that theS630

For Dopes 1 to 3 a water precipitation bath (quench) was used Dope 4used a solvent quench which consisted of 45% PEG-200, 45% water and 10%NMP. The following settings were used for production: Dope Pump SpeedLumen Pump Speed Winder pump speed Dope 1 150 400 300 Dope 2 350 200 800Dope 3 350 200 800 Dope 4 250 170 645

For dopes 1 and 4, as soon as the fibers came out of the quench and ontothe winder, they were divided into the six post-treatment solutionsoutlined below and immersed until testing took place:

-   1. 3% Tetra ethylenepentamine (TEP)-   2. 3% Tris-(hydroxymethyl) aminomethane (TRIS)-   3. 1% Sulfuric Acid (H₂SO₄)-   4. 100% Polyethylene glycol (PEG)-   5. 100% Calcium Carbonate (CaCO₃)-   6. 100% Butanol

For dopes 2 and 3, the fibers were left on the winder until productionwas complete and then they were divided into the six differentpost-treatments and one stayed in the water as a reference.

Dope Formulations that were Unsuccessful TABLE ## Dope formulations thatgelled. Dope 3 Dope 4 Dope 5 PVDF 122.14 (20.3%) 121.49 (20.1%) 100.56(20%) Gantrez  18.14 (3%)  15.24 (3%) S630  15.94 (2.6%)  15.78 (2.6%)Sugar LiCl  18.65 (3.1%)  19.61 (3.2%)  15.62 (3.1%) NMP 428.29 (71%)449.02 (74.1%)  371.2 (73.9%) Water Total 603.16 605.9 502.62

Dopes 3, 4 and 5 ingredients were all mixed together on normal rollersover weekend.

Quantitive Results on Flat Sheet Membranes

Using a HACH spectrophotometer, Dope 2 formulation was used and thefollowing results were obtained: Before After Before After samples insamples in samples samples Methyl orange Methyl Orange in Saffranin inSaffranin TEP 4.5 3.294 3.354 3.159 TRIS 4.5 3.178 3.243 3.131 PEG-2003.466 3.193 3.243 3.289 CaCO₃ 4.5 3.275 3.216 2.582 Butanol 3.876 3.1783.203 3.132 H₂SO₄ 3.165 3.224 3.392 2.943

Using Dope 3 formulation, the following results were obtained: AfterBefore samples in After membranes membranes in dye Methyl Orange in dyewith buffer tablet pH7 TEP 0.423 0.219 0.185 TRIS 0.429 0.411 0.367PEG-200 0.411 0.390 0.350 Butanol 0.429 0.394 0.373 H₂SO₄ 0.440 0.1390.402 CaCO₃ 0.427 0.369 0.362

Break Extension Results on Hollow Fibre Membranes

The following results are from Dope 1 formulation. All results areexpressed as a percentage break extension by length. Wet Dry TEP 32.32 ±7.65 47.20 ± 4.78 TRIS 34.05 ± 7.12 50.13 ± 9.51 PEG 60.51 ± 8.69 33.62± 5.55 H₂SO₄ 22.57 ± 2.53 21.63 ± 1.56 CaCO₃ 24.19 ± 8.11 26.30 ± 5.11Butanol 30.80 ± 6.57

No Gantrez dope Gantrez Dope Kynar 500 Dope TEP 49.46 ± 9.54 23.64 ±5.13  58.8 ± 6.13 TRIS 54.46 ± 10.32 17.18 ± 2.308 77.75 ± 10.78 PEG90.25 ± 11.60 52.75 ± 4.158 123.0 ± 9.1 H₂SO₄ 48.28 ± 7.44 6.008 ± 0.86847.18 ± 13.13 Na₂CO₃ 40.55 ± 13.93 18.30 ± 2.41 67.74 ± 9.25 Butanol60.86 ± 6.60 8.338 ± 3.415 31.48 ± 3.77 Water 44.85 ± 8.79 7.385 ± 2.973

Permeability Results on Hollow Fiber Membranes Flow Flow LMH time @ time@ # Length OD LMH @ @ 100 kPa 400 kPa fibers (m) (mm) 100 kPa 400 kPaTRIS 39.3 15.0 4 0.6 0.85 143.005 374.672 PEG 36.5 10.4 4 0.6 0.85153.975 540.393 H₂SO₄ 128.2 33.6 4 0.5 0.85 52.606 200.717 Butanol 116.434.3 4 0.4 0.85 72.424 245.776 CaCO₃ 53.4 14.6 4 0.55 0.85 114.813419.931

LMH LMH w/o w # length OD of no of Gantrez Gantrez fibers (m) (μm)Gantrez Gantrez TRIS 127.4 379.3 6 0.45 1000 33.330 11.195 PEG 38.1965.8 6 0.5 1100 91.187 3.597 H2SO4 95.3 3534.1 6 0.5 1000 40.101 1.081Butanol 70.7 6 0.5 1000 54.055 NaCO₃ 84.7 6 0.55 1000 41.018 TEP 123.9 60.55 1000 28.041 Water 275.7 6 0.5 1000 13.862

1. A porous polymeric membrane formed from a blend of a polymericmembrane forming material and an anhydride as a reactivity modifyingagent adapted to modify at least one surface active property of theporous polymeric membrane relative to a porous polymeric membrane formedfrom the polymeric membrane forming material without said anhydride. 2.The porous polymeric membrane according to claim 1, wherein thepolymeric membrane forming material comprises polyvinylidene fluoride.3. The porous polymeric membrane according to claim 1, wherein thepolymeric membrane forming material comprises hydrophobic polyvinylidenefluoride.
 4. The porous polymeric membrane according to claim 1, whereinthe polymeric membrane forming material comprises a polysulfone.
 5. Theporous polymeric membrane according to claim 4, wherein the polysulfonecomprises a material selected from the group consisting of polysulfone,polyether sulfone, polyaryl sulfone, polyalkyl sulfone, and polyaralkylsulfone.
 6. The porous polymeric membrane according to claim 4, whereinthe polysulfone comprises polyphenyl sulfone.
 7. The porous polymericmembrane according to claim 1, wherein the reactivity modifying agentmodifies a reactivity of a membrane surface to a chemical species. 8.The porous polymeric membrane according to claim 1, wherein thereactivity modifying agent modifies a hydrophilicity/hydrophobicitybalance of the porous polymeric membrane.
 9. The porous polymericmembrane according to claim 1, wherein the reactivity modifying agentmodifies a surface charge of the porous polymeric membrane.
 10. Theporous polymeric membrane according to claim 1, wherein the reactivitymodifying agent comprises a linear polymeric anhydride.
 11. The porouspolymeric membrane according to claim 1, wherein the reactivitymodifying agent comprises a maleic anhydride copolymer.
 12. The porouspolymeric membrane according to claim 1, wherein the reactivitymodifying agent comprises a poly(alkyl vinyl ether/maleic anhydride).13. The porous polymeric membrane according to claim 1, wherein thereactivity modifying agent comprises a monomer of formula:


14. The porous polymeric membrane according to claim 1, wherein thereactivity modifying agent comprises poly(methyl vinyl ether/maleicanhydride) having a molecular weight of from 5×10⁴ to 5×10⁷.
 15. Theporous polymeric membrane according to claim 14, wherein the reactivitymodifying agent comprises poly(methyl vinyl ether/maleic anhydride)having a molecular weight of from 2.13×10⁵ to 1.89×10⁶.
 16. The porouspolymeric membrane according to claim 15, wherein the reactivitymodifying agent comprises poly(methyl vinyl ether/maleic anhydride)having a molecular weight of from 8.72×10⁵ to 1.25×10⁶.
 17. The porouspolymeric membrane according to claim 1, comprising surface anhydridesites.
 18. The porous polymeric membrane according to claim 17, whereinsaid surface anhydride sites are in a reacted form.
 19. The porouspolymeric membrane according to claim 18, wherein said surface anhydridesites are reacted with an amine to form an amide linkage.
 20. The porouspolymeric membrane according to claim 19, wherein said surface anhydridesites are reacted with a polyamine to form a crosslinked polyamidelayer.
 21. The porous polymeric membrane according to claim 1, whereinthere is a negative charge on a surface of the porous polymericmembrane.
 22. The porous polymeric membrane according to claim 1,wherein the porous polymeric membrane has a high permeability.
 23. Theporous polymeric membrane according to claim 1, wherein the porouspolymeric membrane has a substantial macroscopic integrity.
 24. Theporous polymeric membrane according to claim 1, wherein the membranecomprises a hollow fibre.
 25. The porous polymeric membrane according toclaim 1, wherein the membrane comprises a flat sheet.
 26. The porouspolymeric membrane according to claim 1, wherein the membrane comprisesa uniform wall thickness.
 27. The porous polymeric membrane according toclaim 1, wherein the membrane has a high mechanical strength.